1. Field of the Invention
The present invention relates to a photodetector-amplifier circuit and an optical pickup device. More particularly, the present invention relates to a photodetector-amplifier circuit and an optical pickup device which can switch a plurality of photo-detecting sections.
2. Description of the Background Art
Recently, as the packaging density, function, and operating speed of semiconductor integrated circuits have been improved, technology has also been developed in which a semiconductor integrated circuit is composed of a plurality of function circuits, and a function circuit to be operated is selected by switching ON or OFF a current supplied to each function circuit.
As an example of such technology, a photodetector-amplifier circuit will be described which is used in an optical pickup device of an optical disc drive to read a signal from an optical disc medium.
Optical disc drive apparatuses capable of recording data onto optical disc media, such as DVD, CD, and the like, are becoming wide spread. In an optical disc drive apparatus which performs both a data read operation and a data write operation, a laser light power for writing data onto an optical disc medium is different from a laser light power for reading data from the optical disc medium. Therefore, when the laser light powers are changed, the gain of the photodetector-amplifier circuit included in the optical pickup device needs to be switched in order that the output voltage falls within the output dynamic range of the amplifying circuit.
FIG. 7 is a diagram illustrating a conventional photodetector-amplifier circuit disclosed in Japanese Patent Laid-Open Publication No. H10-107563. As illustrated in FIG. 7, the output terminal of a photodetector 72 is connected to the inverting input terminal (−) of an operational amplifier 71. Gain resistors 74 and 75 are each connected in parallel between the inverting input terminal (−) and the output terminal of the operational amplifier 71 to form a negative feedback loop. More specifically, the gain resistor 75 is connected directly to the output terminal of the operational amplifier 71, while the gain resistor 74 is connected via an analog switch 76 to the output terminal of the operational amplifier 71.
According to the conventional photodetector-amplifier circuit of FIG. 7, the gain of the operational amplifier 71 can be switched by turning the analog switch 76 ON or OFF. When data is read from an optical disc medium, the power of laser light emitted from a light source is set to be small. In this case, by turning the analog switch 76 OFF, the gain of the operational amplifier 71 is set to be a high value due to the resistance of the gain resistor 75. On the other hand, when data is written onto an optical disc medium, the power of laser light emitted from the light source is set to be larger than when data is read. In this case, by turning the analog switch 76 ON, the gain of the operational amplifier 71 is regulated by the parallel addition of the resistances of the gain resistors 74 and 75, and therefore, is smaller than when data is read.
Recently, optical disc drive apparatuses capable of reading and writing data with respect to a plurality of types of optical disc media have also been known. For example, an optical disc drive apparatus supporting two disc specifications DVD and CD employs two laser light beams having different wavelengths, i.e., infrared light and red light, as light sources. In this case, an optical pickup device included in the optical disc drive apparatus comprises a plurality of photodetectors supporting respective light source wavelengths, and one of the photodetectors needs to be selected using a switch, depending on the type of an optical disc medium used.
To this end, a photodetector-amplifier circuit has been proposed in which the same number of analog switch circuits as that of photodetectors are provided between the photodetectors and an operational amplifier, and one of the photodetectors to be connected to the input terminal of the operational amplifier is selected, depending on the wavelength of a light source used. According to the photodetector-amplifier circuit thus proposed, the circuit size and the complication of the wiring layout can be reduced as compared to the photodetector-amplifier circuit in which operational amplifiers are separately connected to the respective photodetectors, and a photodetector is selected by turning the operational amplifiers ON or OFF.
The above-described photodetector-amplifier circuit comprising a plurality of photodetectors has the following problem.
The photodetector-amplifier circuit comprising a plurality of photodetectors and one operational amplifier further comprises a plurality of analog switches which connect the respective photodetectors and the input terminal of the amplifier. Using the analog switches, one of the photodetectors which is to be used is selected, and only the selected photodetector is electrically connected to the input terminal of the operational amplifier.
However, since the analog switch has a parasitic capacitance, a current path cannot be completely interrupted even when the switch is OFF. Therefore, when a photoelectric current occurs by stray light entering a photodetector which is not connected to an operational amplifier, the photoelectric current leaks into the operational amplifier via an analog switch which should interrupt the photodetector and the operational amplifier. As a result, the operational amplifier outputs a noise signal, leading to an unstable operation of the photodetector-amplifier circuit.